Recently, it was reported that surficial nanostructures of black silicon, wings of cicadas and dragonflies have a bactericidal activity (Non-patent Document 1). For example, reportedly, black silicon has 500 nm high nanopillars, and the physical structure of the nanopillars produces a bactericidal activity. Wings of cicadas and dragonflies have 240 nm high nanopillars.
According to Non-patent Document 1, black silicon has the strongest bactericidal activity on Gram-negative bacteria, while wings of dragonflies have a weaker bactericidal activity, and wings of cicadas have a still weaker bactericidal activity. The static contact angle (hereinafter, sometimes simply referred to as “contact angle”) of the black silicon surface with respect to water is 800, while the contact angles of the surface of wings of dragonflies and cicadas with respect to water are 153° and 159°, respectively.